The message
from T i m contains these words:

A fault to earth through the negative conductor
will only show when the supply to the appliance is actually being used.
That's the nature of un-balanced current flow in both conductors.

Well I certainly managed to trip the RCD in my sisters house while
working on a disconnected ring main. I put it down to the fact that the
MCB only isolates the live and the neutrals are all linked thus a
neutral/earth short anywhere on the system will give an alternative
return path for any device drawing current.

--
Roger